A microphone is a listening sensor which converts a sound signal to an electrical signal. The MEMS (Microelectromechanical systems) microphone market has been increasing since the last five years due to the successful applications of MEMS microphones in consumer electronics such as mobile phones, personal computers (PCs) and laptops, digital cameras, etc.
There are mainly two kinds of MEMS microphones; one being a parallel plate capacitor based microphone, which listens to the input sound signal and works in an Omni mode or its mechanical out of plane motion mode. As the mainstream MEMS microphones in the consumer electronics market, this type of microphones can listen to the input sound signal very well. However, the microphone listens to all sound signals applied at its diaphragm, including those signals that are undesired, such as background noise caused by wind or traffic around the microphone. In addition, this type of microphones suffers from a stiction (static friction) issue because of its small electrostatic gap (2 μm˜4 μm) between the diaphragm and the back plate, a large diaphragm radius (usually >500 μm) and a small diaphragm thickness (usually <2 μm). An anti-stiction coating, e.g. a self assembled monolayer (SAM), etc., can be used to solve this stiction issue, but it will add to the costs of the microphones.
The other type of microphone is a bio-inspired microphone, which usually is used for sound source localization and works in a directional mode or its mechanical rocking mode. One example is a comb finger based directional microphone, which utilizes an optical measurement method to realize low noise differential detection and sound source localization. Another example is a centrally-supported circular diaphragm based microphone for large sensitivity and exact sound source localization.
Both types of microphones have a common point in that they only work in one mode: the Omni mode or the directional mode, meaning that the microphones either listen to the sound (with no directivity) or judge the direction of the sound source.